The present invention relates to a unique word detecting system in a digital communication system used in a multiphase and/or multilevel modulation system.
A multiphase/multilevel modulation system, such as a quadrature phase shift keying system, an eight-phase phase shift keying system, a sixteen valued QAM (Quadrature Amplitude Modulation) system or a sixty-four-valued QAM system, is used in order to effectively utilize a radio frequency band in a digital communication system. A unique word is widely employed for the purpose of acquisition and maintenance of synchronization, phase ambiguity removal generated by a demodulator, and a signal type recognition and the like in such a digital communication system. In the multiphase or multilevel system, information bits equal to or more than two bits are transmitted in parallel at the same time by one transmission symbol. Accordingly, equal to or more than two bits of the unique word are also transmitted in parallel at the same time by one transmission symbol. When the information bits are transmitted in parallel by L bits per transmission symbol, the number of the bits of the unique word transmitted in parallel is not always L and sometimes is less than L depending on the multiphase/multilevel modulation system. For example, in the case of the quadrature phase shift keying system one transmission symbol transmits two information bits and also two bits of the unique word are transmitted in parallel at the same time by one transmission symbol. On the other hand, in the case of the eight-phase phase shift keying system, while one transmission symbol transmits three information bits, two bits of the unique word are generally transmitted in parallel.
Fig.5 shows the structure of a prior unique word detector employed in a multiphase/multilevel modulation system.
In Fig.5, a reference numeral 1 designates a multiphase or multilevel demodulator, 2 is a parallel/series converter for converting parallel data sequences to a series data sequence, 3 is a shift register, 4 is a correlator and 5 is a threshold judgment circuit. The operation of the unique word detector 100 shown in Fig.5 is briefly described below. A received signal is input to the multiphase/multilevel demodulator 1 and information bits with L bits per transmission symbol are output in parallel. When M bits (M &lt;L) of a unique word are transmitted with one transmission symbol in parallel, these M bits are input to the parallel/series converter 2 to be converted to a series data sequence. At this time, the clock rate is also multiplied by M. Thereafter, the parallel/series-converted data sequence is input to the shift register 3. The shift register 3 is input with a new piece of data at every clock time and shifts the old data to the right bit by bit, resulting in the deletion of the oldest data. The whole data in the shift register 3 which have the same length as the unique word length are input to the correlator 4 in parallel at every clock time and a correlation value with respect to the unique word pattern is computed. When the data in the shift register 3 are a.sub.o, a.sub.1 ...a.sub.N-1 and the unique word patterns are u.sub.0, u.sub.1...u.sub.N-1, the correlation value R is computed by ##EQU1## where a.sub.i and u.sub.i (i=0, 1...N-1) take values 0 or 1, respectively. A symbol .sym. means an exclusive OR, taking the value 0 when a.sub.i and u.sub.i coincide with each other and 1 when they do not. In short, the correlation value R is equal to the number of inconsistent bits in the data of the shift register 3 and the unique word pattern. Such a correlation value is generally called the Hamming distance. The difference between the number of consistent bits and the number of inconsistent bits in the data of the shift register 3 and the unique word pattern is often used as the correlation value. In this case, the correlation value takes either a positive or a negative value. A description will be made hereinafter by using the correlation value that takes the positive or negative value. A difference between the definitions of the two kinds of correlation values is expedient and has no relation with unique word detection characteristics.
The unique word is detected by threshold-judging a correlation value produced at every clock time.
In the unique word detector 100 described above and shown in FIG. 5, however, the clock rate is increased by the parallel/series converter 2 and, as a result, processing in the correlator must be fairly rapidly performed.
In order to solve that problem, a unique word detector 101 whose structure is shown in Fig.6 is widely employed. In FIG. 6, a reference numeral 1 designates a multiphase/multilevel demodulator, 3 (3a-3m) are shift registers, 4 (4a-4m) are correlators, 6 is a combiner and 5 is a threshold judgment circuit. Different from the unique word detector 100 shown in FIG. 5, no parallel/series conversion is conducted, and parallel M bits are input to M numbers of the shift registers 3. In the case of FIG. 6, when the whole unique word length is N bits, the length of each shift register 3 is N/M. The whole data in each shift register 3 are input to the corresponding correlator 4 at every clock time and the correlation value are computed therein. Thereafter, the correlation value for the whole unique word is computed by the combiner 6 and a unique word detection is performed through a threshold judgement by the threshold judgment circuit 5.
For the detailed description of a prior unique word detecting system, the structure of a unique word detector 102 for the quadrature phase shift keying system is illustrated in FIG. 7. It is presumed that the unique word detector also performs the determination of the recovered phase in the demodulator together with the detection of the unique word because phase ambiguity exists in the recovered phase of the quadrature-phase demodulator. In FIG. 7, a reference numeral 7 designates a quadrature demodulator, 3 (3a, 3b) are shift registers, 8 (8.sub.a, 8.sub.b) are p-correlators, 9 (9.sub.a, 9.sub.b) are Q-correlators, 6 is a combiner and 5 is a threshold judgment circuit. The received quadrature phase shift keyed signal is input to the quadrature demodulator 7 to be demodulated therein. Two information bits are output in parallel from the quadrature-phase demodulator 7. Since two parallel output sequences are generally called a P-channel and a Q-channel, the two parallel output sequences as shown in FIG .7 will be called the P-channel and Q-channel hereinafter. P-channel and Q-channel data output from the quadrature-phase demodulator 7 are input to the shift registers 3a and 3b, respectively. The whole data in the shift resisters 3 are input to the P-correlators 8 and the Q-correlators 9 at every clock time. The P-correlators 8 and the Q-correlators 9 compute the correlation value with respect to the unique word pattern transmitted in the P-channel and Q-channel, respectively. Thereafter, four correlator outputs are input to the combiner 6. The combiner 6 computes the sum C.sub.1 of the P-correlator output of the P-channel and the Q-correlator output of the Q-channel and a difference C.sub.2 between the Q-correlator output of the P-channel and the P-correlator output of the Q-channel to give the two computed results C.sub.1 and C.sub.2 to the threshold judgment circuit 5. The threshold judgment circuit 5 detects the unique word when the absolute value of C.sub.1 or C.sub.2 exceeds the threshold value, and recognizes the recovered phase in the quadrature-phase demodulator 7 according to the sign of C.sub.1 or C.sub.2 which exceeds the threshold value.
The operation of the threshold judgment circuit 5 will be described in more detail below. The threshold judgment circuit 5 determines whether the absolute value of C.sub.1 or C.sub.2 exceeds a single threshold value. A determination can also be possible by setting positive and negative threshold values for C.sub.1 and C.sub.2. When the absolute value of C.sub.1 or C.sub.2 is judged to exceed the threshold values, the recovered phase in the quadrature demodulator 7 is determined by which of C.sub.1 and C.sub.2 exceeds the threshold value, and by whether a polarity is positive or negative. Then a unique word detection pulse and a recovered phase determination result are output. The threshold value is set as to satisfy the specifications for a miss detection probability wherein the unique word can not be detected due to error when the unique word exists and for a false detection probability wherein the unique word is detected in error when no unique word exists. Frequently, the unique word patterns transmitted by the P-channel and Q-channel are designed the same as each other or the P-channel pattern of the unique word is the inverted pattern of the Q-channel unique word bit by bit. In that case, the unique word detector 102 shown in FIG. 7 can be simplified by omitting the Q-correlator 9 as shown in FIG. 8.
In some unique word detectors, not the correlation value resulted from subtracting the number of the inconsistent bits from the number of the consistent bits, but the number of inconsistent bits (the Hamming distance) or the number of the consistent bits is used in the correlator to detect the unique word. Even in such a case, the operation of the threshold judgment circuit 5 is equivalent. Therefore, unique word detection characteristics are the same.
The following problems, however, are present in the conventional systems described above:
The threshold value is so selected that the miss and false detection probabilities are less than the specified values. Generally, when the threshold value is made larger, the miss detection probability is decreased and, conversely, the false detection probability is increased. Accordingly, when the threshold value is T, the miss detection probability is larger than the specified value and the false detection probability is smaller than the corresponding specified value, and when the threshold value is T +.DELTA.T (.DELTA.T: the unit of an increase or a decrease in the threshold value), the miss detection probability is smaller than the specified value, and the false detection probability is larger than the specified value, it is determined that the unique word pattern can not satisfy the specification and the unique word with a longer sequence must be used. As a result, the circuit scales of the unique word detectors 100, 101, 102 or 103 must be enlarged and the transmission efficiency of the digital communication system is decreased.
In the conventional systems, when the number of errors contained in the received unique word pattern becomes large, the unique word can not be detected. For example, in the quadrature phase shift keying system described above, if the whole unique word length is N, and the error of N/4 bits occurs, the absolute values of the outputs C.sub.1 and C.sub.2 is sometimes equal to each other and, in this case, the detection of the unique word itself is not possible. In that case, the threshold value is designed to be smaller than N/4. When the miss detection probability is larger than the specified value, the unique word with a longer sequence must be used. As a result, the circuit scales of the unique word detectors 100, 101, 102 or 103 must be enlarged and the transmission efficiency of the digital communication is decreased.